1. Field of the Invention
This invention relates the control of a telecommunications network which includes at least one local exchange connected by routes to one or more parent trunk exchanges leach being one of a plurality of trunk exchanges interconnected by trunk routes and to telecommunications networks so controlled. It is particularly concerned with the detection of focused overloads in such networks.
2. Related Art
Commonly a local exchange is associated with a home exchange, through which incoming calls to the local exchange are routed, and a security exchange, through which outward calls from the local exchange are routed, in normal operation. The security exchange is so called because it can also be used to route incoming calls to the local exchange if the home exchange fails. The security and home exchanges are collectively referred to as the parent exchanges of the local exchange. Exchanges other than the parent exchanges are referred to as far-end trunk exchanges of the local exchanges.
Near real-time network traffic management (NTM) is an essential component of network management if optimal traffic performance in terms of call throughput is to be ensured. To give an indication of the volume of traffic which may be involved, BT's trunk network in the United Kingdom currently handles approximately six million call attempts per hour during the busy periods which is equivalent to 1,700 call attempts per second. Given such a volume of traffic it is essential that any network difficulties are detected and controlled as quickly as possible. For example, difficulties are often encountered by network traffic managers due to abnormal traffic patterns which can be caused by events such as phone-ins, tele-votes and public holidays (for example Christmas Day and New Year's Eve/Day). In all these cases traffic in the network varies widely from the normal level, sometimes quite spectacularly, and the network must be controlled to maintain the best overall network performance.
With the introduction of digital switches such as System X it is possible to monitor closely the performance of each exchange and the routes between them and to the subscribers. BT's Network Traffic Management System (NTMS) currently receives statistics on upwards of 37,000 routes from 490 exchanges in the UK every five minutes, which measurement period was chosen to be a long enough period to be able to obtain a statistically reliable measurement of the network performance whilst being short enough to allow effective real-time control of the network.
The information received by the NTMS is processed to provide CCITT recommended parameters. The parameter values are compared with thresholds to determine if any difficulties exist on the monitored network elements.
Usually the first indication of a network problem is when an `exception` is displayed on a wall-board, or on a graphical interface at an individual manager's workstation, at a Traffic Management Center. Exceptions are those parameter values, calculated from network element measurements, which deviate sufficiently from a predetermined threshold for that value. The exceptions are ranked in a priority order with the top 20 displayed. However, due to the manner in which the thresholds are set by the network traffic managers, some exceptions do not necessarily indicate a difficulty as thresholds are percentage-based and set a value which ensures all potential difficulties are captured. This results in exceptions being displayed that are occasionally spurious or insignificant. The exceptions therefore need to be examined in more detail to determine if a real difficulty exists and whether it warrants any action. To help in this activity several information sources are currently used by the network traffic managers.
The NTMS provides near real-time surveillance and monitoring of the network's status and performance. It provides the network traffic managers with information to enable them to take prompt action to control the flow of traffic to ensure the maximum utilization of the network in all situations. The NTMS allows network traffic managers to look at the raw statistics as well as derived generic parameters and to compare traffic patterns over the last few measurement periods to isolate any trends.
An On-Line Traffic Information System (OTIS) takes the measurement of statistics from the NTMS system and processes them to provide summarised historical data for daily and weekly traffic patterns. This system allows the network traffic managers to examine historical traffic patterns to detect any radical shifts in traffic.
A data management system provides the network traffic managers with an up-to-date copy of the routing tables at all trunk exchanges. This information is used to check the routes to which calls can be routed, which controls are in force and the routing algorithms being used.
There is also a broadcast speaker facility which connects the world-wide network management center to all the regional centers.
Once a potential difficulty has been detected, acknowledged and analyzed, it is characterised and a decision made whether to control it using the available range of expansive and restrictive controls to either allow alternative traffic paths through the network or to restrict and block call attempts to particular areas, respectively. The situation must then be monitored to ensure the controls are having the desired effect and that they are removed as soon as a problem has been dealt with effectively.
It is important that the controls applied are sufficient to suppress the problem yet not so severe that they cause their own problems by congesting other parts of the network or by blocking potentially successful calls.
One class of exception associated with telecommunications networks is the focused overload. The All Circuits Engaged (ACE) CCITT parameter may be monitored in order to determine when a focused overload occurs but this has been found to be not very satisfactory at present and will be less so in the near future when call failures will only be shown (on a wallboard) with respect to the last of several routes tried. Currently, all the routes tried are shown and focused overloads give rise to a graphic "star" effect on a wallboard.